1. Field of Invention
The present invention relates to technical fields of electro-optical devices and electronic apparatuses. More particularly, the invention relates to an electro-optical device including a pair of substrates which sandwich an electro-optical material, such as a liquid crystal, and various lines provided on the substrates, and to an electronic apparatus including such an electro-optical device.
2. Description of Related Art
The related art includes an xe2x80x9cactive matrix electro-optical device,xe2x80x9d which includes an electro-optical material, such as a liquid crystal, pixel electrodes arrayed in a matrix, thin film transistors (hereinafter xe2x80x9cTFTsxe2x80x9d) connected to the respective pixel electrodes, and scanning lines and data lines which are connected to the respective TFTs, the scanning lines extending in parallel in the row direction, the data lines extending in parallel in the column direction, thus enabling active matrix driving.
In addition to a TFT array substrate having the construction described above, such an electro-optical device includes a counter substrate provided with a common electrode which is opposed to the TFT array substrate with the electro-optical material, such as a liquid crystal, therebetween. Other structures that are provided on the TFT array substrate in addition to the TFTs, scanning lines, data lines, etc., include storage capacitors connected to the TFTs in order to store electric fields applied to the pixel electrodes for a predetermined period of time, interlayer insulating films to prevent short circuiting between the individual components, and contact holes formed in the interlayer insulating films in order to provide necessary electrical connection between the individual elements.
However, the related art electro-optical device is subject to a drawback. That is, the TFT has a relatively short lifetime. The reason for this is that, if moisture enters the semiconductor layer and the gate insulating film constituting the TFT, positive charges are generated due to the diffusion of water molecules into the interface between the gate insulating film and the semiconductor layer, resulting in an increase in the threshold voltage Vth in a relatively short period of time. Such a phenomenon is more common in a p-channel TFT.
If the TFT has a relatively short lifetime as described above, the entire electro-optical device is inevitably affected, and a degradation in image quality is observed from a relatively early stage, which may result in device failure.
Such an inconvenience becomes more serious when an electro-optical device is used in a high-temperature, high-humidity environment. The reason for this is that the chance of moisture entry into TFTs increases. When an electro-optical device is used as a light valve for a liquid crystal projector, since relatively strong light emitted from a light source built in the liquid crystal projector is applied to the electro-optical device or the light valve, it is subjected to high temperature. This is a severer environment from the viewpoint of TFT life, and the problem described above easily becomes noticeable.
Additionally, although the TFT array substrate is provided with various components, such as TFTs, scanning lines, and data lines, and interlayer insulating films to isolate these components from each other, even the interlayer insulating films are not capable of sufficiently preventing moisture from entering the TFTs.
The present invention addresses the above and/or other problems, and provides an electro-optical device in which entry of moisture into TFTs is reduced or minimized to prolong the operational life. The invention also provides an electronic apparatus including such an electro-optical device.
In one aspect of the present invention, in order to address or overcome the above, an electro-optical device includes thin film transistors corresponding to intersections of scanning lines and data lines above a first substrate, pixel electrodes corresponding to the thin film transistors, and a nitride film disposed at least on the surfaces of the data lines.
In the electro-optical device of the present invention, the operation of the thin film transistors is controlled through the scanning lines, and image signals are applied to the pixel electrodes through the thin film transistors, thereby enabling so-called xe2x80x9cactive matrix drivingxe2x80x9d.
In particular, in the present invention, since the nitride film is disposed at least on the surfaces of the data lines, the following effect can be obtained. That is, it is possible to reduce or prevent moisture from entering the thin film transistors, or gate insulating films and semiconductor layers constituting the thin film transistors. The reason for this is that the nitride film has a dense structure.
Consequently, in the electro-optical device of the present invention, a stable operation can be achieved for a relatively long period of time.
In the present invention, the nitride film must be formed at least on the surfaces of the data lines, which means that the nitride film may be formed on the scanning lines, or in some cases, may be formed over the entire surface of the substrate. The xe2x80x9cnitride filmxe2x80x9d in the present invention is represented by a silicon nitride film (SiN film, SiON film, or the like). Of course, a nitride film other than this is acceptable.
In one exemplary embodiment of the present invention, in the electro-optical device, the pixel electrodes are arrayed in a matrix, the scanning lines and the data lines intersect with each other corresponding to the matrix of the pixel electrodes, and the nitride film is disposed at least over the surfaces of the data lines and the scanning lines.
In this exemplary embodiment, since the scanning lines and the data lines are formed in a grid pattern or the like as a whole, the nitride film disposed thereon may also be formed in a grid pattern. Consequently, the effect of preventing moisture entry can be obtained more reliably.
The fact that the nitride film can be formed in such a pattern means that it is possible to have a construction in which the nitride film is not substantially placed over the entire surfaces of the pixel electrodes. In such a case, the overall transmittance of the electro-optical device can be maintained. Consequently, in this exemplary embodiment, due to the presence of the nitride film, in addition to the effect of prolonging life, it is possible to provide a brighter image with high quality. According to the research by the present inventors, it has been confirmed that the transmittance decreases by approximately 4% when the nitride film is left over the entire surface compared with the case in which no nitride film is provided.
Additionally, xe2x80x9cbeing arrayed in a matrixxe2x80x9d is a broad concept which includes a structure in which the individual pixel rows and the individual pixel columns extend straight vertically and horizontally, respectively, and also includes a structure in which the individual pixel rows and the individual pixel columns are two-dimensionally arrayed in a meandering manner or in a staggered manner. Therefore, it is to be understood that xe2x80x9cthe shape corresponding to the matrixxe2x80x9d must be construed bearing the above in mind.
In another exemplary embodiment of the present invention, in the electro-optical device, the nitride film is formed in the periphery of an image display region that is defined by the region including the pixel electrodes, the scanning lines, and the data lines.
In this exemplary embodiment, the nitride film is also formed in the periphery of the image display region in addition to at least on the surfaces of the data lines or at least on the surfaces of the data lines and the scanning lines. Consequently, the effect of reducing or preventing moisture entry can be obtained more reliably.
In this exemplary embodiment, more preferably, the nitride film extends only on the data lines in addition to in the periphery of the image display region.
In such a case, as described above, since it is possible to have a structure in which the nitride film is not substantially placed over the entire surfaces of the pixel electrodes, the overall transmittance of the electro-optical device can be maintained.
In the structure in which the nitride film is formed only on the data lines, it is possible to decrease stress which affects internally as is obvious when compared to the structure, for example, that the nitride film is formed over the entire surface of the substrate. Consequently, it is possible to prevent the nitride film itself from being broken due to its internal stress or to reduce such breakage. It is also possible to prevent cracks from occurring in the other components (e.g., interlayer insulating films) around the nitride film under the influence of the internal stress or to reduce such cracks. The same effects are also applied in general to the nitride film with a grid pattern or the like.
Moreover, the present inventors have confirmed that even in the structure in which the nitride film is formed in the periphery of the image display region and only on the data lines, it is possible to prolong the operational life of the thin film transistors or the electro-optical device by approximately three times. Consequently, in this exemplary embodiment, by a reduced or the minimum use of the nitride film, moisture can be effectively reduced or prevented from entering the TFTs.
Additionally, the presence of the nitride film xe2x80x9conly on the data linesxe2x80x9d refers to a case in which the nitride film is placed only right above the data lines, and also refers to a case in which the nitride film is placed in the periphery of and right above the data lines. That is, any case in which the nitride film is placed substantially in the periphery of the data lines only is included in the range of this structure.
In another exemplary embodiment of the present invention, in the electro-optical device, the nitride film is formed in a region excluding light transmission regions above the substrate.
In this exemplary embodiment, first, xe2x80x9cthe light transmission regionsxe2x80x9d refer to regions in which light that contributes to image display passes through the electro-optical device. More specifically, the regions in which the pixel electrodes arrayed in a matrix are formed substantially correspond to the light transmission regions. In other words, the light transmission regions refer to regions excluding the regions in which the scanning lines and the data lines are formed and the light shielding regions in which light-shielding films are formed in a grid pattern on the surface of the substrate.
Consequently, in this exemplary embodiment, since the nitride film is formed in the regions in which light that contributes to image display is not prevented from traveling, there is no or substantially no possibility of a decrease in the overall transmittance of the electro-optical device.
In another exemplary embodiment of the present invention, in the electro-optical device, the width of the nitride film on the data line is larger than the width of the data line.
In this exemplary embodiment, it is possible to reduce damage to the data lines which may occur in the fabrication process of the electro-optical device.
That is, for example, in the case in which the nitride film is formed only on the data lines, specifically, a fabrication process using so-called photolithography is typically assumed, in which after an original nitride film is formed over the entire surface of the substrate, a resist layer having a predetermined pattern (in this case, xe2x80x9ca pattern covering the data lines onlyxe2x80x9d) is formed, and the resist layer and the original film are etched. However, since the etching step and the resist-stripping step are involved in this process, the data lines may be damaged unnecessarily in such steps.
In this exemplary embodiment, however, since the width of the nitride film is larger than the width of the data line, the edges of the nitride film are subjected to damage due to etching, etc., and damage to the data lines can be reduced or minimized.
Consequently, the stable operation of the electro-optical device is ensured, and high-quality images can be displayed.
More preferably, the width of the nitride film is larger than the width of the data line by 0.1 to 2.2 xcexcm for each edge.
In such a construction, the width of the nitride film is properly set with respect to the width of the data line, and the effect of reducing or preventing damage to the data line is achieved more reliably.
In another exemplary embodiment of the present invention, in the electro-optical device, the thickness of the nitride film is 3 to 100 nm.
In this exemplary embodiment, the thickness of the nitride film is properly set, and the influence of the internal stress of the nitride film can be reduced or eliminated more effectively.
If the thickness of the nitride film is set at a relatively small value as described above, the following effect is also obtained. That is, when some components, such as elements, lines, and the like, are formed on the substrate and interlayer insulating films, etc., are formed in the regions in which the components are formed and in the other regions, so-called xe2x80x9cstepsxe2x80x9d may occur on the surfaces of the interlayer insulating films, etc. This is because each component has its specific xe2x80x9cheightxe2x80x9d. If such steps occur, the coating of the alignment layers which are usually provided in an electro-optical device, such as a liquid crystal display, becomes nonuniform, or rubbing treatment cannot be performed properly on the alignment layers, resulting in a degradation in image quality, such as a decrease in contrast.
In this exemplary embodiment of the invention, since the thickness of the nitride film is limited to a relatively small value of approximately 3 to 100 nm, the height of the steps can be reduced or minimized, and the possibility of a decrease in contrast or the like can be reduced.
In another exemplary embodiment of the present invention, the electro-optical device further includes a second substrate which is opposed to the first substrate with an electro-optical material therebetween, and a light-shielding film disposed above the second substrate at a position corresponding to the scanning lines and the data lines. The width of the nitride film is smaller than the width of the light-shielding film.
In this exemplary embodiment, the width of the nitride film is smaller than the width of the light-shielding film. That is, in a plan view, the nitride film is covered with the light-shielding film. Herein, the light-shielding film usually reduces or prevents light transmission so that mixture of light between pixels is avoided and the contrast of an image is enhanced. Therefore, by disposing the nitride film so as to be covered with the light-shielding film, the overall transmittance of the electro-optical device can be properly maintained.
Although the light-shielding film is formed on xe2x80x9cthe second substratexe2x80x9d in this exemplary embodiment, the present invention is not limited thereto. For example, a construction may be designed in which instead of the light-shielding film in this exemplary embodiment, another light-shielding film may be provided on the first substrate (not on xe2x80x9cthe second substratexe2x80x9d). In such a case, as described above, the TFTs, storage capacitors, scanning lines, and data lines, etc., and the interlayer insulating films isolating these components from each other are provided above the first substrate, and another light-shielding film is considered to be one of such components. By placing another light-shielding film between the interlayer insulating films, it may be considered as a so-called embedded light-shielding film.
This exemplary embodiment of the present invention is of course applicable to an electro-optical device which includes a shielding film provided above the second substrate and another shielding film provided above the first substrate.
More preferably, the width of the nitride film is smaller than the width of the light-shielding film by 1 xcexcm or less at each edge.
In such a construction, the width of the nitride film is properly set with respect to the width of the light-shielding film, and the effect of maintaining the transmittance of the electro-optical device is achieved more reliably.
In another exemplary embodiment of the present invention, the electro-optical device further includes a second substrate which is opposed to the first substrate with an electro-optical material therebetween, and a light-shielding film disposed above the second substrate at a position corresponding to the scanning lines and the data lines. The width of the nitride film is larger than the width of the light-shielding film.
In this exemplary embodiment, it is possible to reduce flickering in images. Although the exact reason for this is not clear, the intrinsic refractive index of the nitride film is considered to refract incident light passing aside the light-shielding film. That is, light which falls upon the relatively wide sections of the nitride film is refracted by those sections, and the traveling path of the light is changed. The light which is supposed to enter the thin film transistors is diverted to somewhere else. Consequently, in this exemplary embodiment, it is possible to reduce incident light to the thin film transistors, thereby reducing photoelectric leakage current, resulting in a reduction in flickering.
From the viewpoint described above, the larger width of the nitride film is considered to be advantageous. However, if the width of the nitride film becomes too large, the nitride film extends to the light transmission regions, and the overall transmittance of the electro-optical device is decreased, resulting in a degradation in image quality. Therefore, there is a limitation in the extent of the difference in width between the nitride film and the light-shielding film, and more specifically, the distance between an edge of the light-shielding film and an edge of the nitride film in the same side is preferably approximately 1.7 xcexcm.
It should be understood that the effects of the present invention are also achieved in the structure in which the width of the nitride film is set larger than the width of the data line. In such a case, the light to be refracted corresponds to xe2x80x9clight passing aside the data linesxe2x80x9d.
In another exemplary embodiment of the present invention, the nitride film is formed by a plasma enhanced CVD process.
In this exemplary embodiment, the nitride film of the present invention can be properly formed on the data lines which are usually composed of aluminum or the like to ensure high electric conductivity. Since the melting point of aluminum is low, if the nitride film is formed by a process which requires a high-temperature environment, there is a possibility that the data lines are melted.
In this exemplary embodiment, since the nitride film is formed by a plasma enhanced CVD process, the formation of the nitride film can be performed in a relatively low-temperature environment, and the inconvenience described above does not occur.
In another aspect of the present invention, in order to address or overcome the above, an electronic apparatus includes the electro-optical device of the present invention (or any one of the exemplary embodiments) described above.
In accordance with the present invention, since the electronic apparatuses include the electro-optical devices described above, it is possible to provide various types of electronic apparatuses, such as projection displays (liquid crystal projectors), liquid crystal televisions, mobile phones, electronic pocket diaries, word processors, viewfinder type or monitor-direct-view type video tape recorders, workstations, television telephones, POS terminals, and touch panels, for example, in which images of high quality can be displayed over a relatively long period of time.
The effects described above and further advantages of the present invention will become apparent from the following description of the exemplary embodiments.